The present invention relates to a method and an apparatus for disinfecting hot water, particularly in recirculating systems such as sanitary hot water distribution systems, cooling towers, etc.
As is known, microorganisms that are dangerous for human health, so called pathogens, may dwell in hot water systems for hygienic and sanitary uses or technical and industrial applications.
More recently, it has been discovered that a severe disease, caused by bacteria that are present in hot water, is legionellosis, which affects human beings with a high mortality rate, even above 30%, especially in hospital environments.
Various species of Legionella are known that can attack human beings; however, the most widespread and dangerous one appears to be Legionella Pneumophila, which is responsible for severe respiratory disorders, with symptoms similar to pneumonia, but very dangerous especially for immunosuppressed subjects.
In nature, those bacteria occur in water and at moderate temperatures, not higher than 30° C., and their concentration is very limited and is to be considered normal.
The situation in hot water, up to 65° C. approximately, is different, because in those cases Legionella proliferates significantly, reaching concentrations that are dangerous for human health.
Therefore, hot water used by human beings for hygienic and technical uses, at 45-50° C., in baths, showers, cooling towers, etc., can become a dangerous source of contamination.
Those phenomena have become apparent mainly in hotels, hospitals, ships, spas and the like, where hot water is kept within the optimal conditions for the proliferation of the Legionella microbe.
Faced with a problem whose extent is cause for concern, various organizations, including the World Health Organization, the European Union, the United States' CDC (Centers for Disease Control and Prevention), have prepared guidelines for preventing such phenomenon.
The scientific literature on the subject reports several methods for contrasting Legionella effectively and attempting to sanitize contaminated systems.
Some of the best known are listed hereafter.
Formally, the simplest method appears to be to raise the temperature of the water to 70+80° C. for a few minutes every day, so as to eliminate bacterial colonies by hyperthermia or thermal shock.
However, in the terminal drawing points, that operate discontinuously, the temperature drops locally below 60° C., eliminating the effectiveness of such method.
Another method involves the use of UV lamps. Such method is however difficult to implement in practice because it is difficult to install the lamps. Also, the large number of drawing points, and therefore of lamps, would entail considerable costs that are not always possible to meet.
Among the conventional chemical methods, water chlorination would appear to be useful both for initially purifying the water and for preserving its sterility.
Such conventional method comprises a first step of hyperchlorination, up to 50 mg/l of chlorine, in cold water, and a subsequent step of weaker but continuous chlorination in hot water, from 1 to 3 mg/l.
Unfortunately, such method has revealed the forming of chlorinated organic by-products of confirmed toxicity (THM).
The use of hypochlorites is not recommendable, because the biofilm is not attacked and destroyed significantly.
The use of recent water potabilization techniques, based on the use of chlorine dioxide, an oxidizer that prevents the forming of organic chlorinates, would appear to be more favorable.
Also, it been proposed to use hydrogen peroxide in association with metallic ions; however, in addition to an as yet unproven effectiveness, ion exchange phenomena in the ducts prevent their use in galvanized iron pipes, which are very commonly used in water distribution systems.
According to recent studies, monochloramine appears to be effective in treating potable water against the proliferation of Legionella and in preserving the water's healthiness.
However, the conventional methods implementing the monochloramine have been devised for waterworks and are not suitable for systems where water is at least partially recirculated and are not suitable for dosing the monochloramine into hot water.
As very well known to the expert person, monochloramine decomposes in a much faster way in hot water than in cold water. Therefore, when operating in a recirculation system, it is necessary to restore the optimal level of decomposed monochloramine without accumulating excessive quantities of by-products, namely ammonium ions.
JP-2008 264678 discloses the use of monochloramine for preventing the insurgence of the Legionella bacteria in water in a tank.
Weintraub J. M. et al.: “Legionella reduction after conversion to monochloramine for residual disinfection” American Water Works Association, vol. 100, no. 4, April 2008, pages 129-139 (XP008112647 ISSN: 0003-150X) and Matthew R. Moore et al.: “Introduction of monochloramine into a municipal water system: Impact On Colonization of buildings by Legionella spp.” Applied And Environmental Microbiology, vol. 72, no. 1, January 2006, pages 378-383 (DOI: 10.1128), disclose disinfecting methods for water systems.